(supplementary) The *upward* nod of acknowledgement has quite different connotations to the *downward* nod of acknowledgement.

The user of the *upward* nod of acknowledgement displays the nostrils and their contents to the subject, whilst symultaneously looking down on them. It therefore signals disrespect.
The *downward* nodder looks up to the subject, displaying submission to them. It therefore signals respect.

The nod of acknowledgement is a standard among guys as a way to let the person know that you see them, without having to resort to using words. Girls will generally not accept the nod of acknowledgement and opt for a wave of the hand and/or a smile, perhaps even accompanied by words. The nod of acknowledgement is a wordless conversation.

The only acceptable response to a nod of acknowledgement is another nod. If you speak, you have broken the point of such a gesture.

Situations in which the nod of acknowledgement may be used:
(1) Walking past a friend to class
(2) Catching a friend's eye across the room to say "hi" without verbally saying it
(3) Giving approval of a friend's actions

The all fabled "nod of acknowledgement is an interesting phenomenon. It is, put simply a slight head nod exchanged between males. While its use is documented among females, it is a rare and condition specific phenomenon. While exchanged between friends who, for whatever reason may not want to acknowledge each other verbally, in some places of the world, it is a common greeting among strangers. For example, in the southern United States, it is very common to offer a head nod to another male as you pass each other or get caught glancing at each other. However, as you travel further north, the head nod disappears among unfamiliars. While taking a stroll in NYC, you'd be hard pressed to get any unfamiliar male to return a nod of acknowledgement while simply walking down the street.

Gary and his girlfriend step into the crowded living room, the party in full swing. They notice Tim and his girlfriend. Gary and Tim exchange a nod of acknowledgement across the loud and crowded room, and then return to scanning the party. The two girlfriends do something along the lines of squealing or waving or running to each other and immediately compliment each other's shoes.

2. Dale walks down the street. He passes some guy, and they exchange a nod of acknowledgement.

3. As Lars stands in line at the hardware store scoping out the others waiting in line, he inadvertently catches the eye of Sven, two isles over. They simply exchange a nod of acknowledgement and look away.

go by the above definition, but the nod of acknowledgement is sometimes accompanied by a slight raising of the eyebrows and a slight grin...kind of like "ahh, yes. i see what you are doing. good job. good job."

friend 1 sees friend 2 trying to chat up a girl from the other side of the room. friend one then proceeds to nod his head, with eyebrows slightly raised, and grin slightly grinning.
Friend 2: (thinking) Jim can see me talking to this girl...sweet. he's nodding and raising the eyebrows. sweet (nods back)
Friend 1: (nod of acknowledgement towards friend 1) (thinking) Mmmhmm. That girl is hot.

This has also been spoken as the "guy nod" due to the fact that only guys seem to be the ones that do this, and at ages from middle school on up.

Nod of acknowledgement can be seen when walking through the halls of a school, I guy sees a girl that he knows, and gives her the guy nod to acknowledge the fact that he sees her, but not to draw too much attention to either himself or the situation that's occuring

I usually refer to this as the "Man Nod", since it is generally used by Men with Men. I find the nod is used most often when signalling hello to an oncoming car on a lonely country road, when communicating a silent hello to a male pedestrian from a car, or in any situation where one man wants to acknowledge another man with respect non-audibly. In the case of car-on-car nodding, sometimes lifting 1 finger up to say "Hi" from the wheel often suffices too.

Driving on a desolate country road without seeing anyone for hours, I gave the only driver that passed the 'nod of acknowledgement {man nod}' as he passed and he did the same right back.